It's been deadly, but this season is not a pandemic.

Flu viruses can shift rapidly, making them harder to vaccinate and tough to predict from year to year. That's part of the reason drug makers have such a tough time coming up with a "universal" vaccine for the flu that would protect you for life.

What counts as an "epidemic" changes a little from season to season — and even week to week — but the CDC's latest numbers indicate we've reached epidemic levels, meaning the flu is spreading quickly and leading to more fatal cases than usually expected.

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Pandemic flu happens "when a new flu virus emerges that can infect people and spread globally" quickly and efficiently, the CDC says.

The flu reaches epidemic levels (like we're experiencing now) at some point every season. But pandemic flu levels are even more dangerous.

Long before the 2009 swine flu pandemic, there was the deadly 1918 Spanish flu, which spread around the world, infecting about one in every five people and killing as many as 50 million. That's more casualties than in World War I.

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Though flu levels are still high in 39 states, the CDC's flu chief, Dan Jernigan, says there are signs it has already peaked in some spots.

Experts say what's especially unusual about this year is that the flu is hitting everyone at the same time.

"We often see different parts of the country light up at different times, but for the past three weeks, the entire country has been experiencing lots of flu all at the same time," Jernigan told reporters on Friday.

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But this year is proving to be a tough one for some baby boomers, too.

People from 50 to 64 years old have the second-highest hospitalization rate in the US after elderly people. They're sicker than kids up to age 4 this year, which experts say is unusual.

Coming down with the flu can also increase your risk of heart attack sixfold, a new study found, because it puts more stress on your system and can increase inflammation. Plus, "when you get an infection, your heart is beating faster," Jeffrey Kwong, the study's lead author, told Reuters.

For older adults, there's the more potent higher-dose Fluzone vaccine, as well as Fluad and Flublok, which also pack an extra punch.

As many as 56,000 people die from the flu every year in the US, the CDC reports, many of them older with compromised immune systems.

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People with the flu can be contagious the day before they start showing signs — meaning they don't even know they're spreading the virus around.

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Scientists say influenza can incubate in the body for one to four days before a person shows any signs of getting sick. That means you can be contagious and not know it, as the CDC points out. And some people can spread the virus around without showing any symptoms at all.

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Stay 6 feet away from people with the flu.

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Person-to-person transmission — from talking, coughing, sneezing, or even just breathing on someone else — is more likely to cause an infection than simply touching a surface with some flu virus on it and then reaching for your nose or your mouth.

Despite what you may have heard about flu shots being less effective this year, the vaccine can still help.

It's still a good idea to get vaccinated if you haven't, because it's effective at preventing other strains of the virus. Flu season can last into May, so if you haven't gotten your dose yet, it's not too late.

A 2017 study found that a flu shot could significantly reduce a child's likelihood of dying from the virus.

The shot makes it less likely you'll contract the flu and can reduce the intensity if you do catch it.

Most people who get the flu don't need to see a doctor. If the illness is diagnosed in the first 48 hours, prescription drugs can help make it shorter and milder.

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Many people coming down with the flu can stay home and wait it out with the help of liquids, over-the-counter drugs, and a heavy dose of sleep.

But when prescribed quickly, drugs like Tamiflu can help shorten the illness. There have been some isolated shortages reported at pharmacies in cities like San Francisco, New York, and Atlanta, so it's best to call ahead.

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If you do get sick, there's one simple way to know whether it's safe to head back into the world.

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Experts say you can head back to work or school when you've been fever-free for a full 24 hours.

Typically, people have the flu for about a week, but some people can stay sick with the illness for up to two weeks, and kids can often be contagious for more than seven days with symptoms like fever, chills, congestion, fatigue, cough, and general misery.

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Even dogs aren't immune to this year's flu.

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The canine flu can't spread to humans, but it can leap from dog to dog pretty easily.

Some pet owners vaccinate their pooches with a flu shot, but many vets say it's better to monitor your dog for signs of sickness, as the illness isn't usually deadly and is not widespread in the US.